elskardio

Re: New 990 Discrete Opamp - 100% SMD version
« Reply #20 on: March 23, 2020, 08:58:03 AM »
Can your variation on the 990 drive 75ohm at +/- 24VDC?
No SMD transistor will tolerate that amount of heat!
As mentioned in my first post, my SMD version will happily drive 150ohm at +/- 18VDC.


Re: New 990 Discrete Opamp - 100% SMD version
« Reply #21 on: March 23, 2020, 12:53:49 PM »
No SMD transistor will tolerate that amount of heat!
As mentioned in my first post, my SMD version will happily drive 150ohm at +/- 18VDC.

I figured as much, which is why I asked.

Once you start changing input transistors, and output transistors, heat dissipation capacity, output power, you have a different animal in my opinion.  Those are pretty big changes to the DNA of Deane Jensen's design choices and performance specs.  You could call yours a 990 mini me, derivative, or little 3rd cousin I suppose.

Curious about the audio file comparisons you mentioned.  Nice work Gabriel.

Adam

elskardio

Re: New 990 Discrete Opamp - 100% SMD version
« Reply #22 on: March 23, 2020, 01:27:01 PM »
Once you start changing input transistors, and output transistors, heat dissipation capacity, output power, you have a different animal in my opinion.  Those are pretty big changes to the DNA of Deane Jensen's design choices and performance specs.

Nothing will compete against the original through hole design and that was never my intention. If you really need +/- 24VDC into 75ohm, just stick to the original design and please put heatsinks on those poor output transistors.

You can call my SMD version a Mid-Power 990 if you want. But for most application (500 series modules) it will match the performance of the original and the sonic difference is indistinguishable (to my ears and 4 studio colleagues).

JohnRoberts

Re: New 990 Discrete Opamp - 100% SMD version
« Reply #23 on: March 23, 2020, 02:07:03 PM »
I do not wish to encourage this exercise but wakefield makes a heatsink that supposedly works with those devices rated at 55'C/W to ambient.

https://www.mouser.com/datasheet/2/433/217-844229.pdf

Even less enthusiastic, if you really need to drive 75 ohms to +/-24v you could take a page from audio power amp design and use a class G/H output stage topology.

Whatever...

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

john12ax7

Re: New 990 Discrete Opamp - 100% SMD version
« Reply #24 on: March 23, 2020, 02:56:37 PM »
Being able to drive 75 ohms shouldn't be a requirement.  But you might want to have some current limiting in case someone tries it,,as two 600 ohm loads and a 1:2 transformer gets you there.

JohnRoberts

Re: New 990 Discrete Opamp - 100% SMD version
« Reply #25 on: March 24, 2020, 10:01:22 AM »
Being able to drive 75 ohms shouldn't be a requirement.  But you might want to have some current limiting in case someone tries it,,as two 600 ohm loads and a 1:2 transformer gets you there.

The device dissipation is a longer term effect so perhaps a soft PS rail that is allowed to collapse under high current draw, could support transiently driving peaks at 75 ohm, without releasing smoke.
===
Back last century I made a headphone amp that would happily drive loudspeakers, until the thermal breaker in the wall wart opened up. I made an artificially soft PS (resistor in series with rectifier diode) so under heavy load the PS sagged enough to protect the wall wart when driving loudspeakers.

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.


 

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